Several publications are referenced in this application by numerals in parenthesis in order to more fully describe the state of the art to which this invention pertains. Full citations for these references are found at the end of the specification. The disclosure of each of these publications is incorporated by reference herein.
The basis of acquired, specific immunity in an organism is the ability to discriminate between self and non-self antigenic substances. The mammalian immune system uses cell surface molecules known as the major histocompatibility complex (MHC) for discriminating between self from non-self. There are two classes of MHC molecules: Class I molecules are found on all nucleated cell types in the body; Class II molecules are found mainly on cells involved in producing immune responses. Most specific immune responses are generated against peptides or peptide derivatives associated with MHC molecules.
The structure of MHC molecules is such that they naturally bind small peptides, glycopeptides, phosphopeptides, and the like. One important function of MHC molecules is to bind peptides that are derived from processed products of proteins expressed in cells expressing the MHC molecules, and to transport these to the cell surface for display to the immune system. In this way, some MHC molecules function to expose the immune system to peptides that are representative of normal cellular proteins. This process occurs during development, when self is learned, and continues throughout the organism's lifespan. Different mechanisms of immune tolerance prevent the organism from responding to “self” peptides associated with MHC.
The introduction of non-self proteins into cells results in the appearance of new and different peptides in association with the MHC molecules; these are recognized as “non-self,” resulting in an immune response. For example, viral infection of a cell will result in the production of viral peptides expressed on the surface of antigen-presenting cells in association with MHC molecules (generally Class I MHC). Viral peptides presented with MHC molecules at the cell surface will often be recognized as foreign and an immune response will be mounted. Autoimmune disease can occur if tolerance to some self peptides is lost, or if immune response is produced against viral or other foreign proteins that cross react with normal peptides in the host organism.
In the case of bacterial infections or other insults from external sources, new proteins or compounds enter the organism. Some cells involved in the immune response are capable of phagocytosing foreign organisms or proteins. These immune cells degrade (process) the protein products and the derived peptides are expressed at the cell surface in association with MHC molecules, where a specific adaptive immune response is generated against novel non-self components. This activity is called antigen processing and presentation and cells that mediate this activity are called Antigen Presenting Cells (APC's). Many different immune cell types, including macrophages, dendritic cells, B cells, and other associated cell types, perform this function.
Antigen alone is often insufficient to produce an immune response. Sometimes, antigen must be presented with accompanying “signals” that are mediated by ligand-receptor interactions between the APCs and the responding lymphocytes or between these cells and soluble factors that are present in the surrounding environment. The soluble factors include cytokines and other mediators of inflammation that are usually present at sites of infections or insult (complement, kinins, other growth and cytokine factors). The signals can be positive in nature, resulting in lymphocyte proliferation and generation of an adaptive immune response, or negative in nature, resulting in apoptosis of responding lymphocytes and perhaps immune tolerance to that antigen. Antigen presentation often occurs in the presence of helper T cells or other cell types that secrete arrays of cytokines, which influence or determine the type of immune response that is induced. At a cellular level, specific immune responses are generated in a mixed cellular environment that includes different types of antigen presenting cells, helper T lymphocytes, other types of regulatory cells, and the responding lymphocytes (B cells for antibody responses and T cells for cellular responses). Direct recognition of peptides by T cells can also occur with some cell types, such as allografts, where the allogeneic MHC is directly recognized as foreign.
Antigen processing and its impact on types of immune responses to specific antigens. The mechanism by which antigen is processed and presented and the parameters that determine the types of immune responses that are generated (antibody versus cellular) are at present not well understood for many antigens. It is believed that there are different classes of APCs that can produce different types of immune responses. In general, APC-induced responses to exogenous antigens that are taken up by endocytosis are believed to be presented to the immune system in the context of Class II MHC and lead to recruitment of T helper cells that interact with B cells and ultimately produce an antibody response. In contrast, endogenous peptides from cells associate with MHC Class I molecules and produce cellular activities that include cytotoxic T lymphocytes (CTL) and Delayed Type Hypersensitivity (DTH) T-cells. There are important exceptions to these mechanisms. For example, many CTLs reactive with exogenous peptides have been described, and it is possible to generate CTLs to specific peptides that have been added to in vitro cultures of immune cells.
Other factors can determine the types of immune responses that are generated. For example, the nature of peptide association with MHC (either Class I or Class II) is an important factor that influences types of immune responses. In the case of Class I MHC molecules, there are specific binding motifs for peptide association (Rammensee et al, Ann. Rev. Imm. 11: 213, 1993). Binding motifs have been established for H-2 Kd, Kk, Dd, and other murine and human MHC. There are also parameters of peptide sequence that determine affinity for class II MHC. Thus, the types of peptides to which an individual can mount an immune system response are in part determined by the immunogenetic genotype and phenotype, which establish the shape and structure of the MHC molecules expressed by that individual.
In summary, the types of immune response that are generated in an organism in response to antigenic challenge is the result of a myriad of contributing factors, including: the immunogenetic background of the individual, prior sensitization to antigens, the route and form of antigen exposure, age and gender of the organism, and other factors. Almost all acquired immune responses that involve specific T-cell recognition are directed toward small peptides bound to the peptide binding groove of MHC molecules, the obvious exception being the response to superantigens. Cellular immune reaction (T-helper reaction, CTL, DTH) to peptides bound to MHC are usually generated through presentation of the antigen to T cells by antigen-presenting cells (APCs).
Tumor Vaccines. Cancer cells express aberrant molecules known as tumor-associated antigens. The immune system has the potential to recognize such structures as “foreign” and to mount specific immune responses against them, so as to reject tumor cells in much the same way that an allograft is rejected. This provides the basis for interest in the development of active specific immunotherapeutic (ASI) agents (cancer “vaccines”) based on cancer-associated antigens.
Early studies on rodent tumors induced by chemical carcinogens, ultraviolet radiation, or viruses showed induction of immunological rejection of secondary tumor challenge. Subsequent studies on spontaneous tumors showed that these animals were incapable of inducing immune-mediated rejection of the tumor. Although a large number of human tumor-associated antigens have been characterized, most of these are also expressed by some normal cells. Therefore, immunological tolerance to such molecules makes it difficult to stimulate responses against such antigens. Moreover, it is a concern that induction of strong immune responses against self molecules may result in the development of autoimmune disorders. Since tumor-specific antigens are seldom detected in spontaneous cancers, approaches to develop active specific immunotherapy for common cancers, based on tumor-associated antigens, have been viewed with pessimism.
Nonetheless, interest in tumor immunology and the development of ASI in particular has persisted. There are at least four reasons for the current interest in ASI approaches. First, cell-mediated immune responses have been recognized as the key factor in immunological rejection of cancer. T cells recognize processed peptides in association with major histocompatibility complex (MHC) molecules, so intracellular proteins can give rise to peptide targets for cell-mediated responses. Further, since antigen processing and presentation are critical steps in T cell recognition, cancer-associated alterations (in its post-translational processing or levels of expression) of a self protein may result in presentation of novel peptide fragments on cancer cells. Secondly, tumor specific point mutations in certain genes have been characterized in several animal and human cancers. Some of these mutations generate novel peptide fragments that bind MHC molecules resulting in the production of new epitopes for recognition by T cells. This process allows for the induction of specific immune responses against cancer cells carrying such mutations. Third, manipulation of immune responses using cytokines, mutated antigens, and other means have sometimes resulted in tumor rejection even in cases of tumors that express weakly immunogenic antigens. Fourth, some individuals with severe immunodeficiencies have a higher incidence of tumors than the normal population, suggesting that the immune system plays an important role in eliminating some tumors.
Various methods have been utilized for stimulating general immune responses, especially for non-antigenic or weakly antigenic substances of interest. For example, adjuvants, such as complete Freund's and Ribi's, have long been used for this purpose. These adjuvants comprise oily solutions containing components, such as lipopolysaccharides that stimulate generalized immune responses. It is believed that the oils surround a water-soluble antigen, such as a peptide, thereby protecting it from degradation in the body and facilitating phagocytosis and passage through cell membranes of antigen presenting cells.
Another approach to stimulating the immunogenicity of a weakly-antigenic peptide or protein has been to couple the weak antigen to a carrier protein that is known to be a good immunogen. Common carrier proteins include keyhole limpet hemocyanin, fowl gamma-globulin and bovine serum albumin. Alternatively, the immunogencity of a weak antigen may be enhanced by polymerizing it into large aggregates by way of cross-linking agents, such as glutaraldehyde. Both these methods rest on the notion that a weak antigen coupled to a strong antigen will enhance the generalized immune response. In a similar method, solid-phase resins and peptide synthetic methods may be employed to synthesize a peptide repeatedly, to form a highly-branched complex. Again, the basis for this approach is to present the antigen in very unusual (and very “non-self”) context to the immune system, to stimulate antibody production.
In yet another approach, a weakly antigenic protein or peptide is attached to a solid particle such as a latex bead or resin. The purpose of this approach is to enhance phagocytosis of the antigen by macrophages. Additionally, peptides and proteins have been encapsulated in liposomes to enhance passage through membranes of antigen presenting cells, to enhance phagocytosis and to stimulate generalized immune responses because of the “non-self” characteristics of the liposome carrier.
The approaches described above have met with varying degrees of success in stimulating the immunogenicity of weakly antigenic or non-antigenic substances. However, they provide only a generalized stimulation of immunity, and are not designed to target specific populations of immune system cells (such as antigen presenting cells). A desired objective in effecting therapeutic internvention in various disease states is to provide a means for specifically targeting a protein or peptide to a population of antigen-presenting cells and thereby stimulate those cells to internalize the antigen of interest and present it to the immune system in an effective, specific context. Insofar as it is known, such a system is not currently available.